1. Field of the Invention
The present invention relates to an optical communication system to be utilized in a subscriber communication network.
2. Description of the Background Art
A conventional optical subscriber communication system for providing one service has a typical configuration as shown in FIG. 1A.
In this configuration, a system station side has an optical transmitter 1 of an intensity modulation type which transmits the intensity modulated light signals containing the data to be supplied to the subscribers of the system, with the wavelength of .lambda.1, through an optical fiber 2 to an optical star coupler 3. These intensity modulated light signals are then distributed by the optical star coupler to optical fibers 4, 5, and 6 connected to the subscriber terminals equipped with optical receivers 7, 8, and 9 of a direct detection type which are capable of receiving the intensity modulated light signals with the wavelength .lambda.1. This type of a subscriber communication system can be utilized as a cable TV network system, for example.
Now, consider a case of expanding the service of the system by additionally providing the second service different from the first service provided between the optical transmitter 1 and the optical receivers 7, 8, and 9, in parallel to the first service, only to those subscribers of the first service who also subscribed to the second service as well. Conventionally, there are two types of modification of the configuration of FIG. 1A for adapting the system to such a case. In the following, those elements which are substantially equivalent to those in the configuration of FIG. 1A will be given the same reference numerals in the drawings and their description will be omitted.
One type of modification is to modify the configuration of FIG. 1A to that shown in FIG. 1B, which utilizes the wavelength division multiplexing transmission mode.
Namely, in the configuration shown in FIG. 1B, the system station side further includes a second service optical transmitter 10 of an intensity modulation type which transmits the intensity modulated light signals containing the data of the second service, with the wavelength of .lambda.2 different from the wavelength .lambda.1 of the light signals of the first service. In this case, the light signals of the first service transmitted from the first service optical transmitter 1 and the light signals of the second service transmitted from the second service optical transmitter 10 are multiplexed by an optical wavelength division multiplexer 11 and the multiplexed light signals obtained at the optical wavelength division multiplexer 11 are transmitted through the optical fiber 2 and distributed by the optical star coupler 3 to the optical fibers 4, 5, and 6.
In addition, in this case, the subscriber terminals are also equipped with optical wavelength division demultiplexer 12, 13, and 14 attached on the optical fibers 4, 5, and 6 in front of the first service optical receivers 7, 8 and 9, respectively, which are capable of demultiplexing the light signals of the first service with the wavelength .lambda.1 and the light signals of the second service with the wavelength .lambda.2.
Furthermore, the subscriber terminal of a subscriber who also subscribed to the second service is also equipped with a second service optical receiver 15 of a direct detection type which is capable of receiving the intensity modulated light signals with the wavelength .lambda.2, which receives the light signals of the second service demultiplexed from the light signals of the first service at the optical wavelength division demultiplexer 14.
In this type of modification, it is necessary to provide the optical wavelength division demultiplexers to all the subscriber terminals including the subscriber terminals of those subscribers who do not subscribe to the second service. Consequently, when this type of modification is adopted, the addition of the second service will require the change of the subscriber terminal equipment to those subscribers who do not subscribe to the second service as well, so that this type of modification can be quite inconvenient for a large number of the subscribers of the system.
Another type of modification is to modify the configuration of FIG. 1A to that shown in FIG. 1C, which utilizes an additional optical fiber 17 dedicated to the second service. In this case, the second service optical transmitter 10 at the system station side is directly connected with a separate second service optical receiver 16 additionally provided to the subscriber who subscribe to the second service in addition to the first service through this additional optical fiber 17, independently from the optical fiber network for the first service. Here, in this case, the light signals of the second service may have the same wavelength .lambda.1 as the light signals of the first service, as they are not going to be multiplexed. Also, in this case, when a number of subscribers who subscribe to the second service increases, the distribution of the light signals of the second service using the optical star coupler will be necessary, just as in the optical fiber network for the first service shown in FIG. 1A.
In this type of modification, the already existing optical fiber network for the first service cannot be utilized at all for the additional second service and it is necessary to provide a completely new optical fiber network dedicated to the second service, so that this type of modification can be quite expensive and highly ineconomical.